The entire disclosure of Japanese Patent Application No. 2001-349811 filed on Nov. 15, 2001 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a machine tool, such as a machining center, which performs machining while moving a work table and a main spindle for a tool relative to each other.
2. Description of Related Art
In a machine tool, such as a machining center, a moving object, such as a table or a spindle head, is supported linearly movable on a structure, such as a bed or a frame, via a sliding or rolling guide surface, and is driven (relatively moved) by a drive line which causes a relative movement in the direction of the guide surface.
In this type of machine tool, the positional relationship between the guide surface and the center of gravity of the moving object is not taken into account, and the center of gravity of the moving object and the guide surface are not in the same plane parallel to the direction of movement, but they are located at relatively remote positions. Thus, when the moving object begins or stops movement by the action of the drive line, a flexural vibrating force, which deforms the structure, is increased by the inertial force of the moving object and a reaction force generated in the structure by the drive line. As a result, vibrations are liable to occur.
If machining is started before settlement of the vibrations, the surface roughness of a workpiece increases (namely, its machining precision decreases). Thus, it has been customary practice to slow down acceleration or deceleration when the moving object begins or stops movement; or to start machining after the vibrations settle. Hence, it has been impossible to shorten the machining time and thereby increase productivity.
Techniques for achieving fast acceleration and deceleration in machine tools have been proposed, for example, by Japanese Unexamined Patent Publication No. 1996-318445 and Japanese Unexamined Patent Publication No. 1999-235631. The former technique uses a linear motor as a drive source in a box frame type machine tool. The latter technique gives movement, in a direction perpendicular to each of the moving directions of first and second slides of a main spindle, as movement of a work table, whereby a uniform feeding load is exerted on a pair of drive mechanisms for driving the first slide of the main spindle, no matter what position is taken by the main spindle for a tool which moves toward and away from a workpiece on the first slide. However, neither of these techniques takes into consideration the positional relationship between a pair of guide surfaces and the center of gravity of a moving object. Hence, the aforementioned problems remain unsolved.
The present invention has been accomplished in light of the above-mentioned problems with the conventional machine tool. Its object is to provide a machine tool which can effectively suppress vibrations of a structure generated during movement of a moving object relative to the structure by a drive line, thereby shortening the machining time and increasing productivity.
According to the present invention, there is provided a machine tool comprising:
a moving object movable along guide surfaces provided in a structure; and
a drive line for moving the moving object, and wherein
the guide surfaces are provided at positions in point symmetry about a position of a center of gravity of the moving object, and
the drive line is provided at a position consistent with the position of the center of gravity of the moving object, or at positions in point symmetry about the position of the center of gravity of the moving object.
According to the machine tool of the present invention, vibrations of the structure, which occur when the moving object is moved relative to the structure by the drive line, can be effectively suppressed, and the machining time can be shortened to increase productivity.
In the machine tool, the position of the center of gravity of the moving object, the guide surfaces, and the drive line may be installed in the same plane parallel to the direction of movement of the moving object. In this case, the effects of the present invention can be further enhanced.
In the machine tool, a connecting portion of the drive line connected to the moving object may be installed on a line passing the position of the center of gravity of the moving object and pointing in the direction of movement of the moving object. In this case, the conditions for installation of the drive line can be easily satisfied.
In the machine tool, a pair of the connecting portions of the drive line connected to the moving object may be provided, and a midpoint between the pair of the connecting portions may be consistent with the position of the center of gravity of the moving object. In this case, the conditions for installation of the drive line can be easily satisfied.
In the machine tool, four of the guide surfaces may be provided. In this case, the effects of the present invention can be further enhanced, and the conditions for installation of guide members can be easily satisfied.